The invention relates to a method and encoding device for encoding a sequence of m-bit pattern words as a bit-stream comprising a frame comprising corresponding n-bit symbols, with m<n, said frame further comprising an n-bit comma symbol. Encoding of m-bit pattern words to n-bit pattern symbols, with m<n, is known, for instance from 8B/10B symbol mapping in which a number of consecutive 8-bit pattern words are mapped into a bit-stream of 10-bit encoded pattern symbols and vice versa.
8B/10B encoding is often used in scenarios in which comparable resources, which include computational power and space, are available at an encoder and at a corresponding decoder. FIG. 1A provides a prior art example of such a scenario and schematically shows transmitter/receivers 102 and 103 each comprising a encoding device E and a corresponding decoding device D. Both the encoding device E and decoding device D of each transmitter/receiver typically comprise pre-stored tables used for encoding and decoding respectively, and in general decoding of encoded data requires about the same amount of computational resources as encoding the data itself. In this scenario the computational capability of each encoding device E and its corresponding decoding device D is thus substantially the same, and both require roughly the same amount of space, as indicated schematically by the size of the encoders and decoders in FIG. 1A.
Such 8B/10B encoding is particularly adapted for use in scenarios in which there is two-way communication between transmitter/receivers 102 and 103, in particular when the amount of data sent by transmitter/receiver 102 to transmitter/receiver 103 is of a same order of magnitude as the amount of data sent by transmitter/receiver 103 to transmitter receiver 102, as indicated by the size of the arrows, and wherein space and/or computational requirements at the decoding device do not form a substantial limiting factor when implementing the decoding device.
FIG. 1B shows another scenario in which the amount of available space and/or computational resources at an encoder E is much larger than the space and/or computational resources available at a decoder D, and which is an intended scenario for the present invention. In the example shown, the encoder E is part of a pattern streamer 106 of a maskless lithography system, and the decoder D is part of a beam blanker array 107 of a maskless lithography system, wherein the beam blanker array may be adapted for individually blanking beamlets of a plurality of beamlets, said plurality comprising tens of thousands or hundreds of thousands or more beamlets. The amount of data sent by such a pattern streamer 106 to the beam blanker array 107 is typically in the range of tens of gigabits per second or more and is orders of magnitude greater than the two data streams of FIG. 1A, as shown by the size of the arrow. Though the beam blanker array 107 shown in FIG. 1B does not transmit any data to other devices, in an alternative embodiment the beam blanker array may be adapted to transmit some data, e.g. quality control data indicative of properties of the received input bit-stream such as number of comma symbols detected therein, to an external device. In such a case the amount of data sent by the beam blanker array—if any—is orders of magnitude less than the amount sent by the encoder E, and in in general is not encoded at all. Regardless, the amount of space and/or computational resources available at the decoding device D, which is here shown comprised in the beam blanker array 107, is much smaller than at the encoding device E as indicated schematically by the size of the encoding device E and decoding device 107 in FIG. 1B.
An implementation of 8B/10B mapping in which the redundancy provided by extending m bits to n bits is used to define at least one comma symbol which has a unique bit pattern that does not occur in any two consecutive symbol mappings, i.e. pattern symbols, for the pattern words, is described in European Patent no EP 97 763. In this comma-based implementation a number of m-bit patterns are encoded into a data frame which comprises a corresponding number of encoded n-bit symbols which map the m-bit pattern words, as well as a comma symbol to delimit the start or end of the frame. Upon receipt of a bit-stream at a decoder side, the beginning or end of a frame is determined based on detection of a comma symbol in the stream. When frames are received they may thus be synchronized to provide a measure of robustness against bit-skip errors which may occur during transmission of the frame.
Decoding n-bit symbols which have been encoded using 8B/10B encoding requires a substantial amount of computational resources. Though other coding schemes are known which can be decoded using fewer computational resources, these generally have as a drawback that a false comma symbol, i.e. an unintended comma symbol bit sequence, can frequently occur in the output bit-stream, which can throw off frame synchronization at a decoder. A reliable frame synchronization is particularly important to prevent bit-skip errors from accumulating when large numbers of frames are transmitted in this manner. For instance, when the m-bit pattern data comprises data for patterning a wafer in a lithography system, millions of frames are typically transmitted for patterning a single wafer, and accumulation of bit skip errors could result in pattern data being written at the wrong positions on the wafer.
It is an object of the present invention to provide a method and encoding device which at least partially resolves these issues.